The invention relates to an apparatus and method for discovering the location and severity of defects in ferromagnetic oil well casings. More specifically the invention relates to a casing inspection system and method utilizing eddy current principles for determining small defects and other anomalies in oil well casings.
In the oil and gas industry buried pipe is used extensively for transporting hydrocarbon products. Since the exteriors of these casings are not readily accessible for discovering corrosion or other aging factors, it is necessary to inspect the pipe from its interior. Continuing maintenance of the casing by periodic interior inspections reduces emergency repairs, prevents unexpected shutdowns, allows scheduled replacement of unsound casing, and enhances the general overall efficiency of the transport operation.
It is common to inspect the overall soundness of oil well casings and other ferromagnetic material by generating a magnetic field within the member to be inspected. There are many casing parameters that may be probed during such inspections. For example, some inspection devices determine the overall thickness of the casing. Other devices are designed to detect small defects such as pits, holes, cracks and other localized anomalies in the casing. Still other devices are designed to determine whether any given defect is on the internal or external wall of the casing. In order to more accurately determine the soundness of the casing or piping, many existing devices incorporate means for determining combinations of the above referenced parameters.
Casing inspection tools and methods exist utilizing d.c. flux to energize an oil well casing to determine the location of defects therein. Such a device is disclosed in U.S. Pat. No. 3,940,689 which structurally comprises a central d.c. electromagnet with two end pole pieces for producing a magnetic field within the casing, and a pair of receiver arrays disposed between the end pole pieces for detecting discontinuities within the casing walls. In this type of device, utilizing flux leakage measurements, sections of the casing wall being inspected are magnetized with d.c. magnetic flux.
It is known that in an unflawed casing section the casing magnetic flux flows longitudinally in a uniform method. It is also known that a hole in the casing causes a local increase in the magnetic reluctance of the casing. This in turn disturbs the longitudinal flux pattern and thus flux lines bulge from the walls of the casing. This bulging flux or fringing flux is the flux leakage detected by receiver coils located adjacent to the casing wall. Sets of two receiver coils, displaced axially and being flat wound or printed circuits, are used to detect fringing flux about a discontinuity in the differentially casing wall. The d.c. flux produced by the electromagnets normally has a magnitude on the order of 20 gauss. Since the magnetic field has such a large intensity, the receiver coils may comprise a low number of winding turns, on the order of 15 per coil. This requirement of 15 winding turns readily enables the use of the printed circuit coils or flat wound receiver coils.
A major disadvantage in the casing inspection device energized by a d.c. source to discover discontinuities by a magnetic flux fringing in the casing is that the larger the casing the larger the size of the magnetic core required to properly magnetize the casing. Thus, with the increase in size of the magnetic core the casing inspection tool itself becomes proportionally heavier.
U.S. Pat. No. 3,940,689 further discloses a means for detecting whether the casing defect is interior or exterior to the casing wall. This determination is made by using localized eddy current measurements which are sensitive only to inner wall casing defects, since the eddy current penetration is only about one millimeter into the skin of the casing wall. In order to effectively induce the eddy currents in a localized region of the casing wall a second transmitter coil is located parallel to the casing wall and energized to generate a high frequency alternating magnetic field.
The second transmitter coil generating the high frequency alternating magnetic field is disclosed in U.S. Pat. No. 3,940,689 as being disposed within the receiver pad along with the receiver coils. Since a printed circuit coil is utilized in making the flux leakage measurement, placement and alignment of the second receiver coil in the receiver pad in such a manner so as to have the receiver coil measure the high frequency field causes increased manufacturing difficulties.
Measurements of thickness and local pitting of oil well casings are also made using alternating magnetic fields. U.S. Pat. No. 2,992,390, discloses an inspection tool utilizing a magnetic core element having a transmitter coil located thereon. The transmitter coil is driven by an alternating current source and generates an alternating magnetic field in the casing. The magnetic core is laminated to prevent eddy currents from flowing therein. Receiver coils are placed predetermined distances from the transmitter coil to determine general thickness variations, local, pitting, and permeability changes by measuring the energy received at their specific location. The receiver coils are wound about stubs or protrusions from the magnetic core, with each stub located a fixed distance from the transmitter depending upon the parameter to be measured.
Prior d.c. magnetic flux leakage casing measurement tools have a major disadvantage because of their inability to distinguish magnetic anomalies from the real casing defects. Further, the overall weight of these devices may be excessive due to the need for an electromagnetic core. Prior eddy current casing measurement tools have relied on magnetic central cores and multiple frequency excitation with resultant heavy and complicated equipment.
Also, use of flat wound or planar configuration printed circuit coils as receiver coils presents the disadvantage of requiring a magnetic field having a magnitude on the order of 20 gauss due to the limited number of windings per coil.